Lining for cylindrical mills

ABSTRACT

The improvement provides a lining for a cylindrical ball or tube mill partly filled with grinding media, which lining has an inside surface bounding a series of trunco-conical volumes which each have a flare towards the mill inlet, the generatrices of any two consecutive truncated cones forming different angles with the mill axis and therefore, producing small-flare truncated cones alternating with large-flare truncated cones, the minor base of the small-flare cones coinciding with the major base of the large-flare cones, the minor base of the large-flare cones meeting the major base of the small-flare cones in an annular surface which is substantially perpendicular to the mill axis; at the limit, said small-flare cones may be cylinders.

United States Patent Pierre Marie Arsene Slegten [72] Inventor 158 Avenue du Prince dOrange, l 180 Brussels, Belgium [21] Appl. No. 16,828 [22] Filed Mar. 5, 1970 [45] Patented Dec. 28, 1971 [32] Priority Mar. 5, 1969 Will! [31] 729.390

[54] LINING FOR CYLINDRICAL MILLS Primary ExaminerRobert L. Spruill Attomeywaters, Roditi, Schwartz & Nissen ABSTRACT: The improvement provides a lining for a cylindrical ball or tube mill partly filled with grinding media, which lining has an inside surface bounding a series of trunco-conical volumes which each have a flare towards the mill inlet, the generatrices of any two consecutive truncated cones forming different angles with the mill axis and therefore, producing small-flare truncated cones alternating with large-flare truncated cones, the minor base of the small-flare cones coinciding with the major base of the large-flare'cones, the minor base of the large-flare cones meeting the major base of the small-flare cones in an annular surface which is substantially perpendicular to the mill axis; at the limit, said small-flare cones may be cylinders.

Patented Dec. 28, 1971 2 Sheets-Sheet 1 Fig- 6 w PRIOR A RT Patented Dec. 28, 1971 2 Sheets-Sheet 2 LINING FOR CYLINDRICAL MILLS This invention relates to a lining for a cylindrical tube or similar mill.

Linings of this kind generally comprise plates whose inside surface in the mill bounds a series of trunco-conical volumes which are flared towards the mill entry. Linings of this kind are used to provide automatic classification of the grinding media in decreasing order of magnitude from the entry to the exit of the milling chamber housing such media. The dimensions. of the grinding media used with this kind of lining decrease proportionally as the fineness of the material being ground increases; it has been found in a very large number of industrial constructions that this feature increases efficiency considerably, inter alia in cement grinding.

Unfortunately, the grading provided by conventional linings is often found to be either unsatisfactory or nil or even the converse of what is required, and the difficulties increase as the tube mills continuously increase in size; diameters have gradually increased form less than 2 meters to meters or more, accompanied by a simultaneous decrease in the lengthto-diameter ratio which, for instance, in the case of cement, was approximately 6 for a 2-meter mill as compared with approximately 3 for a S-meter mill. Also, in a mill 12 meters long and 2 meters in diameter, the superficial area of lining per ton of load was three times what it is in a mill meters long and 5 meters in diameter. Many more grinding media per unit of lining surface area must, therefore, be graded in large mills than in small mills, yet the grading is required in a tube having a smaller'length-to-diameter ratio than was previously the case, and it is found in industry that even the most sophisticated of the known linings fail to provide satisfactory grading in such cases.

In closed-circuit grading, which is becoming more and more the trend, the substance being ground must pass through the mill more than once and at high speed, and it is found in practice that closed-circuit grinding tends to produce reverse grading of the grinding media, the small ones going to the entry and the large ones going to the exit.

The classifying effect from conventional grading linings, is therefore, very uncertain and it is an object of this invention to provide a self-grading lining providing better grading than the known linings.

According to the present invention, there is provided a lining for a cylindrical mill, which lining has an inside surface bounding a series of trunco-conical volumes which each have a flare towards the mill inlet, the generatrices of any two consecutive truncated cones forming different angles with the mill axis and therefore, producing small-flare truncated cones alternating with large-flare truncated cones, the minor base of the small-flare cones coinciding with the major base of the large-flare cones, the minor base of the large-flare cones meeting the major base of the small-flare cones in an annular surface which is substantially perpendicular to the mill axis.

Preferably, the length of the small-flare cones and the angle formed by their generatrix with the mill axis are adapted to correspond to the action on zone of the annular surfaces on the drive of the grinding media.

Advantageously, the lining may have substantially longitudinal raised parts and recessed parts forming corrugations so directed as, when they descend towards the bottom of the load, not to apply axial thrust to the grinding media.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings, in which:

FIG. 1 shows a vertical section on the longitudinal axis of a mill having lining according to the invention,

FIG. 2 shows a cross-sectional view on the line IIII of FlG.

FIG. 3 shows a partly sectional view on an enlarged scale on the line IlI-Ill of FIG. 4,

FIG. 4 shows a view of the inside surface of lining shown in FIG. 1,

FIG. 5 shows a cross-sectional view on the line VV of FIG. 1, giving a perspective view of the lining element having corrugation, and l FIG. 6 shows a perspective view of a lining element having conventional corrugations.

Referring now to the drawings, FIG. I shows a rotatable cylindrical mill comprising a barrel 1 having an inner covering formed by a lining 2 whose inside surface bounds a series of trunco-conical volumes which are flared towards the mill inlet.

According to the invention, the generatrices of any two consecutive truncated cones form different angles with the longitudinal axis of the mill, producing small-flare truncated cones 3 alternating with large-flare truncated cones 4.

As may be seen from F IG. 3, minor base 5 of the small-flare cones 3 coincides with the major base (which also has the reference 5) of the large-flare cones 4; minor base 6 of the I cones 4 joins major base 7 of cones 3 through an annular surface 8 which is substantially perpendicular to the mill axis.

The mill is rotated in the direction indicated by arrow 9 and is partly filled by grinding media 10 and by material 11 being ground. The material being ground moves in the direction indicated by arrow 12. The grinding media 10 are graded automatically so that their dimension diminishes regularly from the mill inlet towards the mill outlet.

Since the peripheral velocity of the lining inside surface is proportionally greater as the diameter increases, the rotation displaces the grinding media higher-up in the flared part of the small-flare cone 3 than in the reduced part of the large-flare cone 4; this effect is shown diagrammatically in FIG. 2 by a line 13, representing the path of the grinding media in the flared part of small-flare cone 3, and by a line 14, representing the path of the grinding media in the narrow part of large-flare cone 4. As FIG. 1 shows, the apices of the paths have a descending slope 15 in the longitudinal direction between the lines 13 and 14. Close study of grading events has shown that when the grinding media of any path disengage from the wall under the combined effect of centrifugal force and gravity, the resultant of the foreces which are applied to such devices has the surprising and unexpected result of causing some of the media to move towards the exit in the direction indicated by an arrow 16 in FIG. 1. Consequently, and as can be seen in FIG. 2 a depression 17 results at the bottom of the line or path 13. The grinding media fall freely as far as a place 18, where they rejoin the main body of the grinding media; between place I8 and place 19 the grinding media roll on the load along the line of greatest slope which brings them to the depression 17 in the direction indicated by an arrow M11 in FIG. 1. While rolling on the load, the large media which are retarded less than the small media, descend faster than the latter and thus return towards the entry more than do the small media. The small media are therefore, gradually displaced towards the narrow part of the large-flare cone 4.

It has been found in pilot trials that for satisfactory grading, the ideal condition is to have a vigorous deflection 16 towards the mill outlet but without the grinding media being hurled too far, for in such a case the large media could not return satisfactorily towards the entry. It is also found that the deflecting action 16 is linked with cone slope and with the lift effect due to the rotation of the annular surface 8.

These observations underlie the invention for, because the drive action provided by the annular surface is considerable, the inclination hear the annular surface need not be steep to produce an adequate deflection I6, and so the cone 3 may have a small flare. However as the distance from the annular surface 8 increases, the drive decreases and, since a steeper slope is required to produce the deflection 16, a cone 4 having a larger flare is necessary.

Therefore, the length l of the small-flare cone 3 corresponds to approximately the action zone of the annular surface8 on the drive of the grinding media. If h denotes the height of surface 8, 1 depends upon h and is in practice generally somewhere between I: and 2h.

The flare or slope angle a of the small-flare cone 3 relatively to the mill axis is such that the cumulative effect of the drive by the annular surface 8 and of the reduced inclination of the cone 3 produces an optimum deflection 16 in the zone of length I. Since the large-flare cone 4 is outside the action zone of the surface 8, the flare angle [3 of cone 4that is, the inclination thereof relatively to the mill axismay be determined solely in dependence upon mill and load characteristics. The best practical results are obtained with 3 1 5 to 25 and a=il3 to iii/3.

The length L of the large-flare cone 4 should be as great as possible since grading is faster between two sections of the mill which are bounded by two consecutive annular surfaces 8 than from any one of such sections to the next. However, since B is generally at least double a, the value of h increases rapidly with the value of L and an overlarge h is undesirable, since the minor base 6 of the large-flare cone 4 and, therefore, the mean internal diameter of the millthat is, the effective volume thereofwould then diminish excessively. In practice, h should be less than 0.05 D (D denoting the internal diameter of the mill barrel) so that, with B=l5 to 25, there is a top limit for the value of L.

The combination of differently sloped truncated cones 3, 4 whose inclinations a, B and lengths l, L are clearly determined makes it possible according to the invention to provide everywhere in the sections bounded by two annular surfaces 8 better grading than can be provided by conventional linings whose design is based on a less thorough study of grading phenomena, for conventional lining has only a single truncated cone between two annular surfaces 8 and the slope selected for this single cone is ideally suited to only one part of the cone.

Another considerable advantage which may be provided by the reduced inclination of the small-flare cones 3 is the provision of a zone where the mean diameter is relatively large and where rotation of the mill lifts the grinding media higher than if there wasjust a single truncated cone having the same slope as the large-flare cone 4. The increased lift given the grinding media in the cones 3 accentuates the depression 17 which, as already described, brings the large media toward the mill inlet, thus speeding up grading. Since grading proceeds more rapidly in each section bounded by two annular surfaces 8, better grading is also provided from section to section over the whole length of the grinding chamber. Furthermore, the reduced slope of the cones 3 helps to reduce pressure of the grinding media on the annular surfaces 8 and hence to reduce the normally very heavy wear in this area.

It will also be seen that, by using the steep inclinations B only in lining portions where they are usefulnamely for the large-flare cones 4the lining weight can be reduced and the useful volume of the mill increased. The weight saving may be 20 percent as compared with lining comprising truncated cones having a uniform slope which is the same as the slope of the large-flare truncated cone according to the invention.

FOr satisfactory lifting of the grinding media 10, the inside surface of the lining may often, with advantage, have raised parts and recessed parts forming corrugations. In known corrugated grading linings, the crestlines and the valley or trough lines of the corrugations converge toward the mill axis.

F IG. 6, which shows a conventional corrugated grading lining, is a partial section, in a perspective from the mill inlet, showing a plate 21 forming part of the lining and sectioned in its flared portion. Plate 21 is corrugated and the crestlines and trough lines of the corrugations converge toward the mill axis. It is found-and the effect may be seen clearly in FIG. 6that when the sides or flanks 20 of the corrugations penetrate into the grinding media at the bottom of the load as the mill rotates, the flanks form an incident angle with the media 10 such that the same are deflected towards the outlet, whereas what is required in this area of incidence between flanks 20 and media 10 is to make the large media move towards the inlet. There is, therefore, a reverse grading effect in the known facility. This may be tolerable in conventional linings but is unsatisfactory with linings according to the invention, for the same usually form in the large-flare cone 4 an angle B which is larger than the angle of the truncated cones of the conven tional single-slope linings, since the large-flare cone 4 is outside the zone of action or influence of the annular surface 8 and may, therefore, have an increased slope. Since the slope is greater, the angle of incidence of the load-penetrating corrugation flanks 20 is increased correspondingly, and there is a corresponding increase in the disadvantageous effect of such angle, as may be seen in FIG. 6.

To obviate this disadvantage, the corrugations are directed to cancel out the angle of incidence of the flanks 20.

FIG. 5, in which a plate 21 forms part of the lining, shows the corrugations as devised in accordance with the invention. The corrugations are so directed that the flank 20 may penetrate into the bottom of the load parallel thereto and without deflecting the grinding media 10 axially. In practice, the direction of the corrugations depends upon the paths which depend, inter alia, upon the coefficient of mill filling. THe direction of the corrugations may readily be calculated by a skilled addressee, who will be familiar withthe shape of the paths of the mill for which he designs a grading lining.

FIGS. 3 and 4 show further detailed exemplary views of an embodiment of the constituent plates of a lining according to the invention. The mill is drilled to German Industrial Standards with one fixing aperture every 250 mm. of its length, with a row of bolts around its periphery every 3 14 mm. Conveniently in this case, the plates are of a length which is a multiple of 250 mm. In FIGS. 3 and 4, the plates 22 and 23 are approximately 250 mm. long and 314 mm. wide. Each plate is secured to barrel 1 by a bolt 24. Two rings of plates 22 and 23 form inside the mill the two truncated cones 3, 4 which flare towards the mill inlet, the length of cone 3 being approximately equal to the height of the annular surface 8.

The generatrix of the small-flare truncated cone 3 forms an angle a of 7 with the mill axis and the generatrix of the largeflare cone 4 forms an angle B of 18 with the mill axis-that is, the small angle a is l/2.57 of the large angle B. The addedtogether length l+L of the two cones 3 and 4 corresponds to two intervals between the bolts 24 and is therefore 500 mm. In the example selected, the mill diameter is 5 meters and the height h of the annular surface 8 is less than 0.05 times the internal diameter D of the mill.

The lining has corrugations to drive the load as the mill rotates. The crestlines 25 and valley or trough lines 26 of plate 23 and of that part of plate 22 which corresponds to the largeflare truncated cone 4 form an angle of approximately l3 with the longitudinal axis of the plate so as not to produce any angle of incidence with the grinding media at the bottom of the load and to prevent axial deflection of such media. The figure of 13 is calculated in accordance with mill characteristics on the basis of criteria familiar to skilled addressees including speed of rotation, filling factor, etc. The crestlines 25 and trough lines 26 of that part of the plate 22 which corresponds to the small-flare truncated cone 3 converge towards the mill axis for manufacturing reasons. It would, of course, be satisfactory for the lines 25 and 26 of the small-flare truncated cone 3 to form any angle between 0 and l3 with the plate longitudinal axis.

When the speed of mill rotation is intended to be such that there is no need to increase lifting of the load, the plates may be smooth and uncorrugated. If required, the plates may have any desired rough parts or recessed parts which may assist to produce an appropriate lift without impairing grading. The

plates 22 and 23 are usually made of cast iron or steel but may be made of rubber or any other suitable material.

The invention may also be embodied with the provision between the small-flare truncated cones and the large-flare truncated cones of one or more intermediate-flare truncated cones and/or by using, instead of truncated cones, the surface produced by a curved generatrix similar to the sequence of the generatrices of the truncated cones.

It has been found in practice of the invention that in some speed conditions and conditions of filling with grinding media, the angle a may, conveniently, be less than 7 and in the extreme case may be 0, in which event the small-flare cone 3 becomes a cylinder, whereupon 1 may be increased to above 2h. In an experiment in which the small-flare cone was cylindrical, good results were obtained with l=6h.

I claim:

1. In a ball or tube mill partly filled with grinding media, a lining which has an inside surface bounding a series of truncoconical volumes which each having a flare towards the mill inlet, the generatrices of any two consecutive truncated cones forming different angles with the mill axis and therefore, producing small-flare truncated cones alternating with largeflare truncated cones, the minor base of the small-flare cones coinciding with the major base of the large-flare cones, the minor base of the large-flare cones meeting the major base of the small-flare cones in an annular surface which is substantially perpendicular to the mill axis.

2. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the length of said smallflare cones corresponds to the action zone of said annular surface on the drive of said grinding media.

3. In a ball or tube mill partly filled with grinding media, a lining according to claim 2, wherein the angle of said smallflare cones to the mill axis is dependent upon said length of said small-flare cones, upon the height of said annular surface and upon the mill and media load characteristics so that the cumulative effect of the drive by said annular surface and of the reduced inclination of said small-flare cone produces optimum deflection towards the mill outlet of the falling grinding media.

4. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the angle of said largeflare cones to the mill axis is dependent upon the mill and media load characteristics to produce optimum deflection of the falling grinding media in said large-flare cones.

5. In a ball or tube mill partly filled with grinding media, a lining according to claim 4, wherein said angle of said largeflare cones to the mill axis is in the range of from 15 to 25.

6. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the height of said annular surface is less than 0.05 times the internal diameter of the mill barrel.

7. In a ball or tube mill partly filled with grinding media, a lining according to claim 3, wherein said angle of said smallflare cones to the mill axis is substantially greater than 0 and less than half the angle of said large-flare cones to the mill axis.

8. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein said small-flare cones are cylinders.

9. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the length of said smallflare cones is greater than the height of said annular surface.

10. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the inside surface is corrugated and the crestlines and valley lines of the corrugations corresponding to said large-flare cones are so directed that on rotation of said mill the corrugation walls or flanks are presented parallel to the bottom of the media load and do not deflect the same axially, whereas the crestlines and the valley lines of the corrugations corresponding to the small-flare cones converge towards the mill axis.

11. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the inside surface is corrugated and the crestlines and the valley lines of the corrugations corresponding to said large-flare cones are so directed that, on rotation of said mill the walls or flanks are presented parallel to the bottom of the media load and do not deflect the same axially, whereas the crestlines and the valley lines of the corrugations corresponding to said small-flare cones diverge from the mill axis to the same extent as or less than do the crestlines and valley lines corresponding to said large-flare cones.

12. In a ball or tube mill partly filled with grinding media, a lining according to claim 8, wherein the length of said cylinders is greater than the height of said annular surface. 

1. In a ball or tube mill partly filled with grinding media, a lining which has an inside surface bounding a series of truncoconical volumes which each having a flare towards the mill inlet, the generatrices of any two consecutive truncated cones forming different angles with the mill axis and therefore, producing small-flare truncated cones alternating with large-flare truncated cones, the minor base of the small-flare cones coinciding with the major base of the large-flare cones, the minor base of the large-flare cones meeting the major base of the small-flare cones in an annular surface which is substantially perpendicular to the mill axis.
 2. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the length of said small-flare cones corresponds to the action zone of said annular surface on the drive of said grindIng media.
 3. In a ball or tube mill partly filled with grinding media, a lining according to claim 2, wherein the angle of said small-flare cones to the mill axis is dependent upon said length of said small-flare cones, upon the height of said annular surface and upon the mill and media load characteristics so that the cumulative effect of the drive by said annular surface and of the reduced inclination of said small-flare cone produces optimum deflection towards the mill outlet of the falling grinding media.
 4. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the angle of said large-flare cones to the mill axis is dependent upon the mill and media load characteristics to produce optimum deflection of the falling grinding media in said large-flare cones.
 5. In a ball or tube mill partly filled with grinding media, a lining according to claim 4, wherein said angle of said large-flare cones to the mill axis is in the range of from 15* to 25*.
 6. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the height of said annular surface is less than 0.05 times the internal diameter of the mill barrel.
 7. In a ball or tube mill partly filled with grinding media, a lining according to claim 3, wherein said angle of said small-flare cones to the mill axis is substantially greater than 0* and less than half the angle of said large-flare cones to the mill axis.
 8. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein said small-flare cones are cylinders.
 9. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the length of said small-flare cones is greater than the height of said annular surface.
 10. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the inside surface is corrugated and the crestlines and valley lines of the corrugations corresponding to said large-flare cones are so directed that on rotation of said mill the corrugation walls or flanks are presented parallel to the bottom of the media load and do not deflect the same axially, whereas the crestlines and the valley lines of the corrugations corresponding to the small-flare cones converge towards the mill axis.
 11. In a ball or tube mill partly filled with grinding media, a lining according to claim 1, wherein the inside surface is corrugated and the crestlines and the valley lines of the corrugations corresponding to said large-flare cones are so directed that, on rotation of said mill the walls or flanks are presented parallel to the bottom of the media load and do not deflect the same axially, whereas the crestlines and the valley lines of the corrugations corresponding to said small-flare cones diverge from the mill axis to the same extent as or less than do the crestlines and valley lines corresponding to said large-flare cones.
 12. In a ball or tube mill partly filled with grinding media, a lining according to claim 8, wherein the length of said cylinders is greater than the height of said annular surface. 